To stop people from committing mass violence, you have to identify the root causes and then address those. Ideally, relieve poverty and quit teaching that might makes right. But there are lots of smaller-scale things that could be done at the school (or workplace) level and do not require mobilizing the efforts of an entire unwilling government and populace.
If I wanted to stop school shootings and had the power to make real changes, I would:
* Teach the basics of emotional first aid for self and others, first to adults, then to students. Here is a general guide to emotional first aid resources. EFA reduces the chance of small problems festering into larger ones.
* Teach various types of meditation to children and adults. Equanimity in the face of stress is a learned skill. Acquiring it reduces stress of all kinds.
* Teach mediation and conflict-resolution skills at all grade levels, with a particular focus on older students helping younger ones to solve problems.
* Add a good anti-bullying program in concert with the above. Treat bullying as a serious issue and take steps to address it instead of letting students abuse each other with impunity. Here are some individual techniques to reduce bullying.
* When students misbehave, strive to identify the root cause of that. Behavior is communication, especially for younger or less-verbal children. Often what it says is something like "My home life sucks" or "I have no social skills." Connect families with support services as needed. Remember that punishment hurts, discipline teaches. Teach missing skills to students. Apply positive discipline with natural and logical consequences. This is more effective. Explore "101 Positive Principles of Discipline."
* Direct extra attention to students who are struggling, isolated, new to the school, and/or belong to a disadvantaged group. Make sure those with disabilities have accommodations. Take preemptive action when bad things happen, such as a death in the family, to minimize the chance of major problems developing. You may not be able to remove the bad thing, but you can avoid the loneliness and despair.
* Resilience helps people withstand challenges without developing serious problems as a result. Start with the basics at home and at school. Here is a community guide for building resilience.
* Start clubs and programs that promote tolerance, diversity, and rationality. Think Sankofa, Debate Club, Gay-Straight Alliance, Mix It Up at Lunch, STEM, Young Astronauts, etc. Actually teach the skills needed to analyze and solve problems, get along with different cultures, and take advantage of the benefits from a mixed group.
* Involve parents as much as possible. Check their skills too, if they are amenable to that. Offer to fill in any gaps. Somebody in the parent body will probably have most everyday skills; set up a program for them to teach other adults who need it. Pair very young parents with older, more experienced ones.
* Expand the library, adding more materials on healthy problem-solving, accountability, giving back, and gentle fiction about problems that can't be fixed by hitting. You can't go wrong by throwing books at a problem.
* Incorporate a wide range of lesson types for different sensory modes, learning styles, and intelligences so that students can learn in a way that works for them and also approach a topic from many different angles.
* Install a quiet room indoors, and encourage everyone to use it when stressed. Oubliettes do not count.
* Add a healthy touch program. Massage has many benefits; hand, foot, or back rubs all work with students in street clothes. But mostly kids will want hugs. Touch deprivation makes primates go insane, rather quickly and quite severely, and enough of it is fatal. It is no more okay to starve children of touch than to starve them of food; it is a survival need. Healthy touch helps keep people calm and happy, reducing violent urges.
* Teach a wide variety of other coping skills. Encourage students and teachers to use them whenever needed, and prompt others to do so if they forget. Remember to backcheck that each student has at least 3 different coping skills for different situations that actually help. Watch out for teachers who forbid students to use coping skills. Replace those teachers if necessary.
* Install a school garden outdoors. Use it both for lessons and for relaxation.
* For any teacher willing, add living things such as houseplants, indoor gardens, aquaria, or other pets in the classrooms. Responsible students may take turns caring for these. They will learn both practical skills and compassion. Also live things are soothing.
* Put a friendship bench on the playground and train a cohort of interested volunteers how to make friends and reach out to lonely kids. Teach friendship skills.
* Include periods of vigorous physical activity in the beginning, middle, and end of the day. Exercise wears off nervous energy. Lack of it causes numerous health problems and inattention in class. Therefore, recess is essential in schools.
* If necessary for local income levels, provide free breakfast, lunch, and after-school meals for all students. They can't regulate their emotions when they're hungry, let alone learn anything.
* Offer shop, home economics, and other practical life skills in the curriculum. Build things to improve the school, such as coat racks or benches. Include art and music not as mere academics, but as a gateway to lifelong hobbies. That is, teach kids the kind of art and music that people use in everyday life. Make relaxing art and hang it around the school. Hold regular performances for people to enjoy. This teaches community skills and self-sufficiency while helping students feel invested in their school.
* Health class should include self-care and positive relationships as well as basic anatomy. Sex ed should be comprehensive -- and not compulsory, because forcing anyone to talk about sex or look at sex pictures is abuse. Some students may not be ready yet and that is okay. You cannot teach real consent if you violate student boundaries right there in class. Put free condoms and sanitary supplies in the bathrooms.
* Connect youth and elders. Invite senior citizens to class for presentations, or have them run clubs. Take regular field trips to senior centers where interested students can find an older mentor.
* Establish a program of volunteering and other civic action, which is in fact voluntary and rewarded with extra credit in the student's grades so that it is not wage theft. Discuss current events and offer a wide variety of extra-credit assignments addressing them. Are you upset about something that happened? Here are some things you can do to feel better or work on a social issue. This encourages prosocial activity, community ties, and rational thinking over violence.
* Create a system of earned privileges in which responsible behavior sensibly leads to greater responsibility. You want to leave campus for lunch? Demonstrate that you know how to navigate locally, make a purchase, be on time, and mind your manners. Here's your pass. This gives students the opportunity to get what they want, and gives them immediate concrete rewards for doing what other people want.
* A similar system may be used for chores such as cleaning the room or shelving library books. Do some work, earn extra credit and/or privileges.
Students who are well-connected, healthy, and busy will not be inclined to murder people. They will also get into much less trouble in general. Some of these solutions cost money, others don't. Any school could pick some within their means and get started on making the world a better place.
Why aren't people doing this? Because it would create vast, sweeping changes in American society. Most people prefer the status quo, even if it includes an increasing creep of violence, militarization, and the educational prison to punitive prison pipeline. I see differently.